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Hopefully it's not a supply pipe to anything. Copper, pvc, electric tape and whatever...
Where does it go? Just follow it and find out what it supplies. When you find out what it supplies you should be able to either figure it out or post back here.
If you have a handy friend, have him take a look.
Ron

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I think we need a little more info. Following the pipe in both directions will really help. Is it 2" or 4". Does the crawl space smell like anything? It almost looks like someone retrofitted a drain line?